1. Field of the Invention
The present invention relates to an ultrasonic motor and more particularly to an elastic member which is utilized in the ultrasonic motor.
2. Description of the Prior Art
Conventionally, an ultrasonic motor has been proposed for rotating a rotor by using the expansion and contraction generated at a piezoelectric vibrator. This kind of ultrasonic motor is exemplified by Japanese Patent Laid Open Nos. 58(1983)-192474 and 48(1983)-192475. However, there is a drawback involving the presence of substantial noise in these ultrasonic motors which results in wear at a contact surface between the piezoelectric vibrator and the driven rotor. In order to solve that drawback, an ultrasonic motor has been proposed wherein a elastic member is interposed between the piezoelectric vibrator and the rotor. A travelling wave is generated at the elastic member by the piezoelectric vibrator in this conventional ultrasonic motor, thereby rotating the rotor. This kind of the ultrasonic motor is disclosed in Japanese Patent Laid Open No. 61(1986)-19127.
In a conventional ultrasonic motor which generates a travelling wave at an elastic member by a piezoelectric vibrator, it is desirable to maximize the amplitude of the travelling wave generated at the elastic member, because the mechanical output of the rotor is increased and the efficiency of converting electrical input energy to mechanical output energy is improved when the amplitude of the travelling wave is enlarged.
In Japanese Patent Laid Open No. 61(1986)-191278, the amplitude of a travelling wave is enlarged by forming a plurality of projecting portions 63 on the elastic member 61 as shown in FIG. 5 herein. The projecting portions 63 have a parallelogram configuration wherein the side surfaces 61A and 61B are parallel. The amplitude of the travelling wave generated at the elastic member 61 by a piezoelectric member 62 is enlarged by the projecting portions 63 which are integrally vibrated with the elastic member 61. Accordingly, the amplitude of the travelling wave and the mechanical output from a rotor is enlarged in accordance with the size of height h of the projecting portions 63.
However, the projecting portions will not be integrally vibrated with the travelling wave in conventional ultrasonic motors, if the height of the projecting portions and the frequency of the travelling wave become too great. That is, the travelling wave generated at the elastic member is no longer enlarged, and the mechanical output from the rotor may actually be decreased. In other words, the drive frequency at which the elastic member is vibrated should not exceed the natural frequency of the projections, or else the projections will become out of resonance with that drive frequency. It would thus be desirable to be able to maximize the height h while minimizing the accompanying decrease in the natural frequency of the projections. In that way, the amplitude of vibration and the effective drive frequency can be maximized.